1. Field of the Invention
The present invention relates to communication systems and, in particular, to an improved pulse shaping filter.
2. Description of the Related Art
In digital communication systems, data symbols are typically made up of a series of square pulses, in which each pulse is transmitted at an amplitude of one of at least two binary states. The pulses are generally shaped to bandlimit the transmitted signal.
The Nyquist class of pulse shapes exhibit an amplitude of zero at a set of equally spaced time instants and minimize the spectrum required to transmit signals. By precise positioning of the zero points, intersymbol interference (ISI) and the performance degradation associated therewith may be avoided. While Nyquist pulses offer spectral efficiency, overall performance is highly sensitive to pulse shape. Deviation from the Nyquist shape can introduce severe performance degradation.
For example, RF anomalies such as those resulting from SAW and ceramic filters and other anomalies across the desired band are typically not taken into account when pulse shaping. As such, even though the pulses may be nominally “Nyquist pulses,” in practice, severe intersymbol interference may be introduced by the pulse shaping filter. One such filter is the pulse shaping filter used in the user and base station of an IS-95 CDMA (TIA/EIA Interim Standard 95 for Code Division Multiple Access) cellular telephone system Similarly, the IS-136 TDMA (TIA/EIA Interim Standard 136 for Time Division Multiple Access) standard employs a square root raised cosine filter to minimize ISI at the optimal sampling point, with pulse shaping at both the transmit and receive ends. The IS-95 and IS-136 standards are incorporated herein by reference in their entireties as if fully set forth herein.
In neither system, however, do the shaping filters take into account the radio frequency (RF) anomalies introduced into the system. For example, the IS-136 filter(s) are matched only to the digital portion of the receiver and do not take into account distortion introduced by the RF and intermediate frequency (IF) components. As such, there is a need for an improved shaping filter that takes into account RF and IF interference.